Electrophotographic elements containing dithiolium salts

ABSTRACT

The electrophotographic elements contain sensitizer compounds having a dithiolium nucleus comprised of a five-membered, generally aromatic, heterocyclic ring containing two sulfur atoms. The sulfur atoms can be in either a 1,2- or a 1,3relationship to one another, and all carbon atoms in the nucleus can be substituted with such groups as hydrogen, alkyl radicals, amino radicals, mono- or polycyclic aryl radicals and oxygen- or sulfur-containing heterocyclic radicals. Additionally, the dithiolium nucleus can form part of a fused ring aromatic structure with any or all of the rings being substituted with any of the above substituents.

llito Etats not Inventors George A. Reynolds;

Bernard C. Cossar, Rochester; Charles V. Wilson, Pittsford, all of NY.

June 22, 11970 Oct. 26, 197 1 Eastman Kodak Company Rochester, N.Y.

Continuation-impart of application Ser. No. 681,928, Nov. 9, 1967, now abandoned.

App]. No. Filed Patented Assignee ELECTROPHOTOGRAPHIC ELEMENTS CONTAINKNG DHTHIOLIUM SALTS 24 Claims, No Drawings 11.1.8. Cl 96/l.5, 96/16, 96/17, 96/18, 252/501, 260/327 Int. Cl G03g 5/04, 603g 5/06 Eleld ol'Search. 96/1.2,1.3,

[56] References Cited UNITED STATES PATENTS 1,678,832 7/1928 Sheppard 96/107 2,728,668 12/1955 Mochel 95/7 Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, Ill Attorneys-W. H. J. Kline, P. R. Holmes and T. Hiatt ABSTRACT: The electrophotographic elements contain ser sltizer compounds having a dithiolium nucleus comprised of a five-membered, generally aromatic, heterocyclic ring containing two sulfur atoms. The sulfur atoms can be in either a 1,2 or a 1,3-re1ationship to one another, and all carbon atoms in the nucleus can be substituted with such groups as hydrogen, alkyl radicals, amino radicals, monoor polycyclic aryl radicals and oxygenor sulfur-containing heterocyclic radicals. Additionally, the dithiolium nucleus can form part of a fused ring aromatic structure with any or all of the rings being substituted with any of the above substituents.

ELEC'lIllWlPIlllUTUGRAPHHC ELEMENTS CONTAINING Dll'lllllllOlLlllUll/il SAlL'Tfi This application is a continuation-in-part application based on application, Ser. No. 681,982, filed Nov. 9, 1967, now abandoned.

This invention relates to electrophotography, and more particularly, to electrophotosensitizers for photoconductive compositions.

Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, US. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,331; 3,220,833 and many others. Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well known in the art can then be employed to produce a permanent record of the image.

One type of photoconductive insulating structural element particularly useful in electrophotography utilizes a composition containing a photoconductive insulating material dispersed in a resinous material. A unitary electrophotographic element is generally produced in a multilayer type of structure by coating a layer of the photoconductive composition onto a film support previously overcoated with a layer of conducting material or else the photoconductive composition can be coated directly onto a coating support of metal or other suitable conducting material. Such photoconductive compositions show improved speed and/or spectral response, as well as other desired electrophotographic characteristics, when one or more photosensitizing materials or addenda are incorporated into the photoconductive composition. Typical addenda of this latter type are disclosed in U.S. Pat. Nos. 3,250,615; 3,141,770 and 2,987,395. Generally photosensitizing addenda used in photoconductive compositions are incorporated to effect a change in the sensitivity or speed of a particular photoconductive system and/or a change in spectral response characteristics. The mechanism of such sensitization is presently not fully understood; however, the phenomenon is extremely useful.

It is, therefore, an object of this invention to provide novel sensitized photoconductor-containing compositions.

It is a further object of this invention to provide novel electrophotograpliic elements employing new electrophotosem sitizing addenda.

These and other objects and advantages are accomplished in accordance with this invention by the use of sensitizing amounts of dithiolium salts in photoconductor-containing compositions. in particular, it has been discovered that a wide variety of 1,2- and l,3-dithiolium salts are extremely useful in sensitizing photoconductive compositions. Such photoconductive compositions containing the electrophotosensitizers of the present invention can be readily coated on a suitable support in the usual manner of preparing a photoconductor element. Furthermore, the addition of these dithiolium salt sensitizers does not adversely affect other properties of electrophotographic elements such as solid area reproduction characteristics, recovery rate and ability to accept either positive or negative electrostatic charging.

The dithiolium compounds useful in the present invention all have in common a dithiolium nucleus which comprises a five-membered heterocyclic ring which is generally aromatic in nature and which contains two sulfur atoms. The two sulfur atoms can be in either a 1,2- or a l,3-relationship to one another. in addition, the dithiolium nucleus can be substituted at all carbon atoms with such substituents as hydrogen, alkyl radicals of from one to live carbon atoms, substituted amino radicals including ones containing from one to carbon atoms, aryl radicals including phenyl and naphthyl as well as substituted phenyl and naphthyl radicals and heterocyclic radicals having at least one hetero atom selected from the group of oxygen and sulfur and containing up to nine carbon atoms. Furthermore, the dithiolium nucleus can form part of a fused ring aromatic structure, such as benzodithiolium, with either or both rings being substituted with any of the substituents referred to above. Other compounds also useful in this invention include those having two fused ring structures, such as benzodithiolium, linked together by an unsaturated aliphatic radical having from one to five carbon atoms.

The 1,2-dithiolium salts of this invention include such salts having a fused saturated fiveor six-membered ring in the 3,4- position, which fused ring can also be substituted with aralkenyl radicals including substituted aralkenyl radicals having on the aromatic portion such substituents as another aralkenyl radical; alkyl, alkoxy, alkylamino, alkylthio etc. radicals of from one to 10 carbon atoms; nitro radicals; hydroxy radicals; alkylenedioxy radicals and the like. In addition, the 1,2- dithiolium salts of this invention also include 3-substituted- 1,2-dithiolium salts wherein the 3-substituent can be an aryl radical such as phenyl, naphthyl etc. including substituted aryl radicals having a hydroxy, alkoxy, alkylenedioxy or tertiaryamino radical thereon; an aralkenyl radical having two to four carbon atoms in the alkenyl moiety and including substituted aralkenyl; and an aromatic heterocyclic alkenyl radical wherein oxygen is the hetero atom and including the substituted radicals which can contain, among others, aralkenyl groups as mentioned above.

The l,3-dithiolium salts useful in the present invention include l,3-benzodithioliurn salts and 4,5-dihydr0-l,3-dithiolium salts including the corresponding 2-substituted salts wherein the 2-substituent can be an amino radical including alkyl and aryl substituted amino radicals; an alkyl radical of from one to 12 carbon atoms including substituted alkyl radicals; an aryl radical including substituted aryl radicals having such substituents as alkyl groups of from one to five carbon atoms, alkoxy groups of from one to five carbon atoms, alkylamino groups of from two to 10 carbon atoms etc.; an aralkenyl radical wherein the alkenyl moiety contains from two to four carbon atoms and including substituted aralkenyl in which the aromatic portion has such substituents as alkyl, alkoxy, alkylamino, alkylthio etc. radicals all of from one to 10 carbon atoms, nitro radicals, hydroxy radicals, alkylenedioxy radicals etc.; and a heterocyclic alkenyl radical having at least one oxygen or sulfur hetero atom and including such radicals substituted on the heterocyclic moiety with any of the substituents included with the aralkenyl radicals above as well as heterocyclic alkenyl radicals substituted with aralkenyl radicals such as those listed above.

Included among the many types of useful 1,2- and 1,3- dithiolium salts are the following general catagories:

a. 5-aryl-l,2-dithiolium salts substituted at the 3-position with an alkyl radical of from one to five carbon atoms, an aryl radical including substituted aryl radicals containing such substituents as hydroxy, alkoxy or tertiaryamino radicals, an aralkenyl radical in which the aryl moiety can be phenyl, naphthy or an aromatic heterocyclic radical wherein the hetero atom is oxygen and having from two to four carbon atoms in the alkenyl moiety, and wherein the S-aryl radical can be substituted with hydroxy, alkoxy, alkylenedioxy, alkylamino etc., radicals and halogens;

b. 3-alkylthio-l,2-dithiolium salts containing a benzylidene substituted fused saturated ring in the 4,5-position wherein the fused ring can be a fiveor six-membered ring and the benzylidene radical can be substituted with lower alkoxy, alkylamino, styryl etc., radicals as well as with a methylidyne-3-alkylthio-l,Z-dithiolium salt radical containing a fused ring at the 4,5-position, and wherein the 3-alkylthio radical contains from one to five carbon atoms;

c. 2-amino-4-aryl-l,3-dithiolium salts wherein the amino and aryl groups can be substituted with aryl or alkyl radicals;

d. 2-substituted-5-alltyl-l,3-benzodithiolium salts wherein the 2-substituent can be an alkyl radical of one to five carbon atoms, an aryl radical including substituted aryl having substituents as lower alkyl radicals and lower alkoxy radicals of from two to carbon atoms etc., an aralkenyl radical having two to four carbon atoms in the alkenyl moiety, an oxygenor sulfur-containing heterocyclic alkenyl radical having from two to four carbon atoms in the alkenyl moiety and a vinyl radical including Z-substituted 5 vinyl radicals having such substituents as an aryl or an aromatic heterocyclic radical including aryl and hetero cyclic radicals having such substituents as lower alkoxy, alkylamino, nitro, alkylenedioxy, styryl etc., radicals, and wherein the 5-alkyl radical has from one to five carbon atoms;

2-aminoaryl-4-aryl-l,3-dithiolium salts wherein either aryl moiety may be substituted and wherein the nitrogen of the amino group can be substituted with alkyl, hydroxyalkyl or aryl radicals; f. 4,5-dihydro-l,3-dithiolium salts substituted at the 2-position with aryl radicals, styryl radicals, including substituted aryl and styryl radicals, and oxygenor Sulfurcontaining heterocyclic alkenyl radicals;

g. 2-araIkenyl-4-aryll ,3-dithiolium salts wherein the 2-aral kenyl radical is as the aralkenyl radical referred to in (a) above and wherein the 4-aryl radical is the same as the 5- aryl in (a) above; and

h. bis-(S-alkyl-l,3-benzodithiolium) alkene salts.

The acid anions associated with the dithiolium cations to form salts include such anions as perchlorate, chloride, bisulfate, methyl sulfate etc. Useful l,2- and 1,3-dithiolium salts thus include, among others, salts of the cations listed in the table below:

TABLE I Dithiolium Cations l. 3-(2-Hydroxyl -naphthyl)-5-phenyl-l ,2-dithiolium 2. 3-(4-Hydroxy-l-naphthyl)-5-phenyl-l,Z-dithiulium 3. 3-(2,4-Dihydroxyphenyl)-5-phenyl-l ,Z-dithiolium 4. 3-[4-(N-2-Hydroxyethyl-N-methylaminol-Z- ethylphenylLS-phenyl-l ,Z-dithiolium 5. 3-(2,4-Diethoxyphenyl)-5-methylphenyl-l .Z-dithiolium 6. 3-(4-Diethylamino-2-hydroxyphenyl)-5-phenyl-l ,2-

dithiolium 7. 3-(4-Hydroxy-3-ethoxyphenyl)-5-phenyl-l,Z-dithiolium 8. 3-(2.3,4-Trihydroxyphenyl)-5-phenyl-l ,Z-dithiolium 9. 3-(2,4-Dibutoxyphenyl)-5-phenyl-I .2-dithiolium 3-(2,4-Diethoxystyryl)-5phenyl-l .2-dithiolium 3-[2-(5-Styryl-2-furyl)vinylI-S-phenyl-1,Z-dithiolium 3-(2-Methoxystyryl)-5 phenyl-l ,Z-dithiolium 3-(4-Ethoxystyryl)-5-phenyl-l ,2-dithiolium 3-(4-Diethylaminostyryl)-5-phenyl-l ,Z-dithiolium 3-(2,3-Diethoxystyryl)-5-phenyl-l .2-dithiolium 3-(2,3-Dihydroxystyryl)-5-phenyl-l ,2-dithiolium 3-(4-Chlorostyryl)-5-phenyl-l .Z-dithiolium 3-(3,4-Methylenedioxystyryl)5-pheny|-1,2 dithiolium l9. 3-(4-Dimethylaminostyryl)-5-phenyl-l,2 dithiolium 20. 6-p-Diethylaminobenzylidcne-3-methylthiocyclopent[c l-l ,2dithiolium 2l. 7Benzylidene-3-ethylthio-cyclohex[c]-l,2dithi0li- 22 7 (3.4-Diethoxybenzylidene-3-methylthio-cyclohex [c]-l,2-dithiolium 23. 2-Piperidino-4-phenyl-l ,3-dithiolium 24. 2-Dithylamino4-p-tolyl-l,B-dithiolium 25. 2-Diethylamino-4-phenyll ,3-dithiolium 26, 2-[Bis(p-ethoxyphenyl)vinyl]-5-methyl-1,3-

benzodithiolium 27. 2-(2,4-Diethoxystyryl)-5ethyl-l,3-benzodithiolium 28. 2'[2-(5-styryl-2-furyl)vinyl]-5-ethyl-l,3-

benzodithiolium 29, 2-p-tolylvinyl-5-ethyl-l,3-benzodithiolium 30. 2-Furylvinyl-5-methyl-l,3-benzodithiolium 31. Z-Phenyl-S-ethyll ,3-benz0dithiolium 32. Z-p-AnisyI-S-methyll ,3-benzodithiolium 33. 2-Propoxyphenyl-5-methyl-l ,3-benzodithiolium 34. 5-Propyl-2-p-tolyll ,3-benzodithiolium 35 2-m-Dimethylaminophenyl-S-methyl-1,3-

benzodithiolium 36. S-Methyl-Z-phenyl-l ,3-benzodithiolium 37. 2-Styryl-5-propyll ,3-benzodithiolium 38. 2-(p-Ethoxystyryl)-5-ethyll ,3-benzodithiolium 39. 2-(3,4-Diethoxystyryl)-5-methyl-l ,3-benzodithiolium 40. 2-(4-Hydroxy-3-ethoxystyryl )-5-methyll ,3-

benzodithiolium 41. 2-p-Dipropylaminostyryl-5-methyll ,3-benzodithioli- 43. 5-Ethyl2-p-nitrostyryl-l ,3-benzodithiolium 44. 5-Methyl-2-( 3,4-methylenedioxystyryl)- l ,3- benzodithiolium 45. 2-(2-Butoxystyryl)-5-methyl-l,3- benzodithiolium 46. 2-( l-Naphthylvinyl)-5-ethyl-l,3-benz0dithi0lium 47. 2-(4-Styrylstyryl)-5-methyl-l ,3-benzodithiolium 48. 2-[Bis(4-diethylaminophenyl)vinyl ]-5-methyl-l ,3-

benzodithiolium 49. 2-(4-Diethylaminostyryl)-4-phenyl-l ,3-dithiolium 50. 2-(4-Ethoxystyryl)-4-phenyl-l,3-dithiolium 51. 2-(4-Diphenylaminophenyl)-4-phenyl-1,3-dithiolium 52. 2-(4-Diethylaminophenyl)-4-phenyll ,3-dithiolium 53. 2- p-[N-Propyl-N-(2-hydroxymethyl)amino1phenyl -4-phenyl-1,3-dithiolium 54. 2-(2,4-Diethoxystyryl)-4.5-dihydro-l,3-dithiolium 55. 2-(4-Ethoxystyryl)-4,5-dihydrol ,3-dithiolium 56. 2-( l-Naphthylvinyl)-4,5-dihydrol ,3-dithiolium 57. 2-Styryl-4,5-dihydr0- l ,3-dithiolium 58. 2-[2-(5-Styryl-2-furyl)vinyl]-4,5-dihydrol ,3-dithioli- Z-p-Diethylaminostyryl-S-methyl-l ,3-benzodithioli- 59. 244-Dipropylaminostyryl)-4.5dihydrol .3-dithiolium 60. 2.[3-(4,5-Dihydr0- l .3-dithiole-2-yl)l-propenylidene]- 4,5-dihydrol ,3-dithiolium A great variety of 1,2- and l,3-dithiolium salts are valuable, in accordance with this invention, as sensitizing agents for enhancing the sensitivity and extending the spectral range of sensitivity of a wide range of photoconductors. The present dithiolium sensitizers can be used with an inorganic, or an organic, including organometallic, photoconducting material which has little or substantially no persistence of photoconductivity. Exemplary inorganic photoconductors would include such materials as zinc oxide, cadmium sulfide, cadmium selenide and the like. Representative organometallic compounds are the organic derivatives of group lVa and Va metals such as those having at least one amino-aryl group attached to the metal atom. Exemplary organometallic compounds are the triphenyl-p-dialkylaminophenyl derivatives of silicon, germanium, tin and lead and the tri-p-dialkylaminophenyl derivatives of arsenic, antimony, phosphorus and bismuth.

An especially useful class of organic photoconductors is referred to herein as organic amine" photoconductors. Such organic photoconductors have as a common structural feature at least one amino group. Useful organic photoconductors which can be spcctrally sensitized in accordance with this invention include, therefore, arylamine compounds comprising (1 diarylamines such as diphenylamine, dinaphthylamine, N,N-diphenylbcnzidine, N-phenyl-l-naphthylamine; N-phenyl-2-naphthylamine; N,N'-diphenyl-p-phcnylencdiamine; 2- carboxy-5-chloro-4-methoxy-diphenylamine; panilinophenol; N,N'-di-2-naphthyl-p-phenylene-diamine; 4,4'- ben2ylidene-bis-(N,N-diethyl-m-toluidine), those described in Fox US. Pat. No. 3,240,597 issued Mar. 15, 1966 and the like and (2) triarylamines including (a) nonpolymeric triarylamines, such as triphenylamine, N,N,N',N'-tetraphenylm-phenylenediamine; 4acetyltriphenylamine, 4-hexanolytriphenylamine; 4-lauroyltriphenylamine; 4-hexyltriphenylamine, 4'dodecyltriphenylamine, 4,4'-bis(diphenylamino)benzil, 4,4'-bis(diphenyl-amino)benzophenone and wherein T represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear, (e.g., phenyl,

naphthyl, biphenyl, binaphthyl, etc.,), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group having from one to about six carbon atoms (e.g., acetyl, propionyl, butyryl, etc.,), an alkyl group having from one to about six carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.,), an alkoxy group having from one to about six carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy, etc.,), or a nitro group; M represents a mononuclear or polynuclear monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.,); or a substituted monovalent aromatic radical wherein said substituent can comprise a member, such as an acyl group having from one to about six carbon atoms (e.g., acetyl, propionyl, butyryl, etc.,), an allryl group having from one to about six carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.,), an allroxy group having from one to about six carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.,), or a nitro group; O can represent a hydrogen atom, a halogen atom or an aromatic amino group, such as MNi-l-; b represents an integer from one to about 12; and R represents a hydrogen atom, a mononuclear or polynuclear aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.,), a substituted aromatic radical wherein said substituent comprises an alkyl group, an allroity group, an acyl group, or a nitro group, or a poly(4'-vinylphenyl) group which is bonded to the nitrogen atom by a carbon atom of the phenyl group.

Polyarylalltane photoconductors are particularly useful in producing the present invention. Such photoconductors are described in U.S. Pat. No. 3,274,000; French Pat. No. 1,383,461 and in copending application of Seus and Goldman titled Photoconductive Elements Containing Organic Photoconductors Ser. No. 627,857, filed Apr. 3, 1967. These photoconductors include leuco bases of diaryl or triaryl methane dye salts, 1,1,l-triarylalkanes wherein the alkane moiety has at least two carbon atoms and tetraarylmethanes, there being substituted an amine group on at least one of the aryl groups attached to the alkane and methane moieties of the latter two classes of photoconductors which are nonleuco base materials.

Preferred polyarylalkane photoconductors can be represented by the formula:

J?IE G wherein each of D, E and G is an aryl group and J is a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and (fl containing an amino substituent. The aryl groups attached to the central carbon atom are preferably phenyl groups, although naphthyl groups can also be used. Such aryl groups can contain such substituents as alkyl and alltoxy typically having one to eight carbon atoms, hydroxy, halogen, etc., in the ortho, meta or para positions, ortho-substituted phenyl being preferred. The aryl groups can also be joined together or cyclized to form a fluorene moiety, for example. The amino substituent can be represented by the for mula wherein each L can be an alkyl group typically having one to eight carbon atoms, hydrogen atom, an aryl group, or together the necessary atoms to form a heterocyclic amino group typically having five to six atoms in the ring such as morpholino, pyridyl, pyrryl, etc. At least one of D, E and G is preferably pdialkylaminophenyl group. When I is an alkyl group, such an alkyl group more generally has one to seven carbon atoms.

Representative useful polyarylalkane photoconductors include the compounds listed below:

TABLE I1 Photoconductors 1. 4,4-benzylidene-bis(N,N-diethyl-m-toluidine) 2. 4,4"-diamino-4-dimethylamino-Z ',2 "-dimethyl- 13. 4',4"-bis (benzylethylamino)-2',2"-dimethyltriphenylmethane l4. 4',4"-bis(diethylamino)-2',2"-diethoxytriphenylmethane 15. 4,4-bis(dimethylamino)-l ,l,l-triphenylethane l6. l-(4-N,N-dimethylaminophenyl)-1,l-diphenylethane 17. 4-dimethylaminotetraphenylmethane 18. 4-diethylaminotetraphenylmethane Another class of photoconductors useful in this invention are the 4-diarylamino-substituted chalcones. Typical compounds of this type are low molecular weight nonpolymeric ketones having the general formula:

wherein R and R are each stituted phenyl radicals and radical having the formula:

phenyl radicals including subparticularly when R is a phenyl vantageous results are obtained when R, is a phenyl radical including substituted phenyl radicals and where R, is diphenylamino, dimethylamino or hydrogen.

Polymeric photoconductors can also be used with the present sensitizing materials. Typical polymeric photoconduc tors would include the various photoconductive carbazole polymers. Useful materials of this type are disclosed, for example, in U.S. Pat. No. 3,041,165 issued June 26, 1962. Of course, in those instances wherein a polymeric photoconductor is used, a separate polymeric binder can be omitted.

The subject dithiolium sensitizers are used in a photoconductive layer by mixing a sensitizing amount of the sensitizer with the coating composition so that, after thorough mixing, the sensitizer is substantially uniformly distributed throughout the desired layer of the coated element. The amount of sensitizer that can be added to a photoconductive layer to give effective increases in speed can vary widely. The optimum concentration in any given case will vary with the specific photoconductor and sensitizing compound used. In general, substantial speed gains can be obtained where an appropriate sensitizer is added in a concentration range from about 0.0001 to about 30 percent by weight based on the weight of the filmfonning coating composition. Normally, the sensitizer is added to the coating composition in an amount by weight from about 0.1 to about 10 percent by weight of the total coating composition.

Preferred binders for use in preparing the present sensitized photoconductive layers are film-forming polymeric binders having fairly high dielectric strength which are good electrically insulating film-forming vehicles. Materials of this type comprise styrene-butadiene copolymers; silicone resins; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; poly(vinyl acetate); vinylacetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); polyacrylic and methacrylic esters, such as poly(methyl methacrylate), poly(n-butyl methacrylate), poly(isobutyl methacrylate), etc.; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such 7 as poly(ethylenealkaryloxyalkylene terephthalate); phenol formaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly(ethylene glycol-co-bishydroxyethoxyphenyl propane terephthalate); etc. Methods of making resins of this type have been described in the prior art, for example, styrenealkyd resins can be prepared according to the method described in U.S. Pat. Nos. 2,258,423. and 2,358,423. Suitable resins ofthe type contemplated for use in the photoconductive layers of the invention are sold under such trade names as Vitel PE-lOl Cymac, Piccopale 100, Saran F-220 and Lexan 105. Other types of binders which can be used in the photoconductive layers of the invention include such materials as paraffin, mineral waxes, etc.

Solvents of choice for preparing coating compositions of the present invention can include a number of solvents such as benzene; toluene; acetone; 2-butan0ne; chlorinated hydrocarbons, e.g., methylene chloride, ethylene chloride etc.; ethers, e.g., tetrahydrofuran, or mixtures of these solvents etc.

In preparing the coating composition useful results are obtained where the photoconductor substance is present in an amount equal to at least about 1 weight percent of the coating composition. The upper limit in the amount of photoconductor substance present can be widely varied in accordance with usual practice. In those cases where a binder is employed, it is normally required that the photoconductor substance be present in an amount from about I weight percent of the coating composition to about 99 weight percent of the coating composition. A preferred weight range for the photoconductor substance in the coating composition is from about l weight percent to about 60 weight percent.

Coating thicknesses of the photoconductive composition on a support can vary widely. Normally, a coating in the range of about 0.001 inch to about 0.01 inch before drying is useful for the practice of this invention. The preferred range of coating thickness is found to be in the range from about 0.002 inch to about 0.006 inch before drying although useful results can be obtained outside of this range. As previously mentioned, more than one layer may be coated on the support. Good results are obtainable when a first layer containing a photoconductor, a binder and a sensitizer is overcoated with a second layer of a composition containing a photoconductor and a binder. The photoconductor and binder employed in the overcoat can be different than those employed in the first layer.

Suitable supporting materials for coating the photoconductive layers of the present invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, etc.; metal plates, such as aluminum, copper, zinc, brass, and galvanized plates; vapor deposited metal layers such as silver or aluminum and the like. An especially useful conducting support can be prepared by coating a support material such as polyethylene terephthalate with a layer containing a semiconductor dispersed in a resin. Such conducting layers both with and without insulating barrier layers are described in U.S. Pat. No. 3,245,833. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of maleic anhydride and a vinyl acetate polymer. Such kinds of conducting layers and methods for their optimum preparation and use and disclosed in U.S. Pat. Nos. 3,007,901 and 3,267,807.

The elements of the present invention can be employed in any of the well-known electrophotographic processes which require photoconductive layers. One such process is the xerographic process. In a process of this type, an electrophotographic element is given a blanket electrostatic charge by placing the same under a corona discharge which serves to give a uniform charge to the surface of the photoconductive layer of at least 400 volts and preferably at least 500 volts. This charge is retained by the layer owing to the substantial insulating property of the layer, i.e., the low conductivity of the layer in the dark. The electrostatic charge formed on the surface of the photoconducting layer is then selectively dissipated from the surface of the layer by exposure to light through an image-bearing transparency by a conventional exposure operation such as, for example, by contact-printing technique, or by lens projection of an image, etc., to form a latent image in the photoconducting layer. By exposure of the surface in this manner, a charged pattern is created by virtue of the fact that light causes the charge to be conducted away in proportion to the amount of the exposure in a particular area. The charge pattern remaining after exposure is then developed, i.e., rendered visible, by treatment with a medium comprising electrostatically attractable particles having optical density. The developing electrostatically attractable particles can be in the form of a dust, e.g., powder, pigment in a resinous carrier, i.e., toner, or a liquid developer may be used in which the developing particles are carried in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature in such patents, for example, as U.S. Pat. No. 2,297,691 and in Australian Pat. No. 212,315. In process of electrophotographic reproduction such as xerography, by selecting a developing particle which has as one of its components, a lowmelting resin, it is possible to treat the developed photoconductive material with heat and cause the powder to adhere permanently to the surface of the photoconductive layer. In other cases, a transfer of the image formed on the photoconductive layer can be made to a second support, which would then become the final print. Techniques of the type indicated are well known in the art and have been described in a number of U.S. and foreign patents, such as U.S. Pat. Nos. 2,297,691 and 2,551,582 and in RCA Review, vol. 15 (1954), pages 469-484.

The following examples further illustrate the invention as well as several preferred embodiments thereof.

EXAMPLES 1-63 A photoconductive composition containing one of the following photoconductors:

l. triphenylamine 2. l,3,5-triphenylpyrazoline 3. 4,4-bis(diethylamino)-2,2-dimethyltriphenylmethane 4. 2,3,4,5-tetraphenylpyrrole 5. 4,4-bis(diethylamino)benzophenone as the photoconductive material is prepared for coating on a conducting support material by mixing 0.15 parts of the photoconductor with 0.002 parts by weight of one of the dithiolium salts and dissolving the mixture, together with 0.5 parts by weight of a resinous polyester binder, by stirring the mix into dichlorornethane. The resultant mixture is then hand coated on an aluminum-laminated paper support. In all instances, the polyester binder in the coating composition is a copolymer of terephthalic acid and a glycol mixture comprising a 9:1 weight ratio of 2,2'-bis[4-(B-hydroxyethoxy)phenyl] propane and ethylene glycol. The wet coating thickness on the support is 0.004 inch. After drying, a sample of each electrophotographic element is employed in a standard xerographic process which includes charging under a positive corona discharge until the surface potential of the sample, as measured by an electrometer probe, reaches 600 volts. Similarly, a sample of each element is charged under a negative source until the surface potential reaches 600 volts. Each of the samples is then exposed from behind a transparent stepped density gray scale to a 3,0001(. tungsten source of foot-candle illuminance at the point of exposure. The exposure causes reduction of surface potential of the element under each step of the gray scale from its initial potential, V0, to some lower potential, V, the exact value of which depends upon the actual amount of exposure received by each area. The results ofthese measurements are then plotted on a graph of surface potential V versus log exposure for each step. The actual speed of each element can then be expressed in terms of the reciprocal of the exposure required to reduce the surface potential to any fixed arbitrarily assigned value. Numerically, the speed is the quotient of 10 divided by the exposure in meter candle seconds required to reduce the potential by 100 volts. The results of these speed measurements are given in the following tables. A control sample, comprising the resinous binder above and the sensitizer 4-(4-n-amyloxyphenyl)-2,6-bis(4-ethy1phenyl)thiapyrylium perchlorate, has the speed of 14 on the same basis as referred to above when charged positively before exposure. In addition, a sample of each element is tested to determine if an image is formed. Each element is charged to a positive surface potential of 600 volts as above. The charged element is then covered with a transparent sheet bearing a pattern of opaque and light-transmitting areas and exposed as above. The resulting electrostatic latent image is developed by cascading over the surface of the element a mixture of glass beads and negatively charged toner particles. The resultant print is then fixed by heating. Where a good reproduction of the pattern results, it is indicated in the following tables under the heading Image Formed." For ease of reference each table contains a citation to a disclosure of the preparation of the class of compounds referred to in that table The speeds as determined above are shown in the following tables under the heading ofH and D Speed.

TABLE lll.3,5-DTARYL1,2DITHIOL1UM SALTS [Reyn0lds, U.S. application filed concurrently herewith] Table (ontinued 4. 3-[4-(N-2-IIydroxyethyl Yes 3 4O N -eth ylarnino)2- 4 5O 50 methyl-phunyll-5- 5 113 32 phcnyl-l,Q-dithiolium.

5. 3-(2,4 diniethox vphenyD- Yes 3 250 500 S-phcnyl-LZ-dithiolium. 4 120 200 6. 3-(4-dimethy1aininoQ- Yes. 3 4O 40 hydroxyphenyl) -5- 4 250 phenyl-l,2-dithiolium. 5 200 7. 3-(4-hydroxy-3 Yes 3 120 .200

methoxyphenyl) -5- 4 120 80 phenyl-l,2-dithi0liuni. 5 250 100 phenyl)5-phenyl1,2- 3 25 120 dithiolium. 5 100 9--. 3(2,4-dipentyloxy- Yes 3 63 160 dithiolinm.

TABLE 1V.-3-A RALKE NYL-5-ARYL-1,2-1)1T1IIOLI UM SALTS [3-methy1-5-aryl4,2-dithi0linni perchlorate is prepared according to Behringer and (irinnn, Ann. (182, 194 (19114). The corresponding 3- aralkenyl compounds are then prepared by hunting the .i-methyl compound with an aromatic aldehyde in acetic acid and/or acetic anhydride] Image conduo losi- Neath No. (oinpound (lerehlomtes) formed tor tive tive 10 3-(2,4-diniethoxystyryl)-5- Yes.. 3 50 40 phenyl-l,Q-dithiolium. 4 .250 250 11,. 3-[2-(5-styryl2-furyl) Yes 3 50 50 viiiyl]*5-phenyl-1,2 4 100 200 dithioliuni. 5 200 160 12.. 3(2-eth0x \'styryD-S- Yes 3 40 Z25 phenyl-l,Q-tlithiolium. 4 101) 13 3-(4-methoxystyryl)-5 Yes l 200 .100 phcnyl-l,2dithiolium. 4 80 120 14. 3-(4-di1nethyla1nino- Yes 3 160 400 sty1yl)-5-plien ll,2- 4 160 1150 dithiolinni. 5 320 1120 15.. 3-(2,3-diniethoxystyryh- Yes 3 100 32 5-plienyl-1,12-dithioliu1n. 4 100 250 17. 3-(4-chlorostyryl)-5- Yes 3 2O 32 phonyl-1,2-dithioliun1. 4 50 63 18. 3-(BA-mothylenedioxy Yes. 3 (,3 4t) styryl)-5-phenyl-1,2- 4 160 120 dithioliuin. 5 120 80 19.. 3-(4-diethylaminostyryl)- Yes. 3 200 400 5-phenyl-1.2-dithioliu1n. 4 4t) 50 TABLE V 1,2-dithi0lii1rn salts containing a benzylidene substituted, fused saturated ring at the 3,4-p0siti0n [R. Mayer and 11. Hartmann, B011, 117, 1886 (11164)] 11 and D spccd sulfate.

TABLE X.-4,5-DIHYDRO-LS-DITHIOLIUM SALTS [R. Wizinger and D. Durr, Helv. Chim. Acta., 46, 2167 (1963)] H and D speed Image conduc- Posi- Nega- No. Compound (Perchlorates) formed tor tive tive 54... 2-(QA-dimethoxystyryl)- 1 500 400 4,5-dihydro'1,3 dithi- Yes. 100 80 olium. 3 40 16 55 2-(2,3 dimethoxystyryD- Yes. l 80 63 4,5-dihydro-1,3- 5 80 16 dithiolium.

5G 2-(4-n1ethoxystyryl)-4,5- Yo I 160 120 dihydro-l ,3-dithio1ium. 5 a 50 20 57 2-(l-naphthylvinyl)-4,5- 1 120 50 dihydro-l,3-dithioiium. Yes. 2 63 32 58.. 2-styryl-4,5-dihydro-1,3- Y 1 32 20 dithiolium. 3 20 2s 59 2-[2-(5-styry1-2-fury1)- 1 500 630 vinyl] 4,5-dihydro-l,3- Yes :2 250 250 ditliiolium. 3 25 63 60.. 2-(4-dimcthylaminostyryD- 1 200 320 4,5dihydro-l,3- Y J 200 250 dithiolium. 3 160 100 61.. 2-[3-(4,5-dihydro 1,& 1 500 630 dithiolcQ-ylidonetl- Yes. 2 500 400 1,3dithio1ium. 5 160 80 TABLE XI.--2-ARALKENYL4-ARYLl,3-DITHIOLIUM SALTS [The preparation of these compounds is given after this table] The 2-aralkenyl-4-aryl-l, 3-dithiolium salts of table XI are prepared by first forming a 2-methyl-4 -aryl-l,3'dithio1ium salt and heating in the usual manner the Zmethyl compound with an aromatic aldehyde in a solvent such as acetic anhydride, acetic acid or l,2,3-trichloropropane. This active methyl compound is prepared by mixing 3 g. of Zmethylmercapto-4-phenyl-l,B-dithiolium perchlorate [Bull Soc. Chim. France, 437 (1955)], 3 g. of 4,6-dil eto-2,2-dimethyl-1,3- dioxane, 2 ml. of diisopropylethylamine and 50 ml. of ethyl alcohol and heating on a steam bath for one hour. The solution is chilled to yield 2 g. of 2-(2,2-dimethyl-4,6-dioxo-l ,3-dioxan -5-ylidene)-4-phenyl-1,3-dithiole, m.p. 202-203 C. Next 1.5 g. of the diothiole in 25 ml. of hot acetic acid is added to 1 ml. of 70 percent perchloric acid. Carbon dioxide is evolved and a solid separates. The mixture is cooled and the product collected. The 2-methyl-4-phenyl-l,3-dithiolium perchlorate has a melting point of l59160 C.

In addition to the above compounds, useful results are also obtained with 2-(5-methyl-l ,3-benzodithiole-2-yl)methine-S- methyl-l,3-benzodithiolium perchlorate; 2-[3-(5-methyl-1,3 benzodithiole-Z-ylidene)- l -propenyl]-5-methyl-l ,3- benzodithiolium perchlorate; 2-[bis(4-methyloxyphenyl)vinyll-5 methyl-l,3-benzodithiolium perchlorate and other related compounds.

EXAMPLE 64 A dispersion of photoconductive zinc oxide in resinous binder is coated on a paper support. The resultant coating has a zinc oxide to binder ratio of 4:1. The binder used in this dispersion is prepared from a combination of about 9 parts by weight of Pliolite 8-7 (a styrene-butadiene resin manufactured by the Goodyear Tire and Rubber Company) and about 1 part by weight of SR-82 resin (a 60 percent solution of a sil icone resin and toluene manufactured by the General Electric Company). The resultant element formed by the zinc oxide and resinous binder coated on paper support is used as a control element. Next, an identical element is prepared which additionally contains 3-(4-diphenylaminophenyl-S-phenyl dithiolium perchlorate (as sensitizer) per grams of zinc oxide. Both the control element and the sensitized element prepared as described above are subjected to a negative corona discharge until a surface charge of 500 volts is obtained on each of the elements. Immediately thereafter both elements are stored in the dark. After 60 seconds of dark storage, the surface charge of the respective elements is measured again. The charge on the control element is -390 volts and the charge measured on the sensitized element is --430 volts.

EXAMPLE 65 Two elements prepared as described in example 64, Le, the control element and the sensitized element, are again charged to a surface charge of 500 volts. in addition, both elements are exposed to a light source having a wavelength of radiation of 580 nm. After exposure, the time required for the surface charge of the two elements to decay from 500 volts to 50 volts is measured. The control element takes more than 60 seconds. The sensitized element takes only 14.3 seconds to decay to 50 volts. Both of the elements, i.e., the nonsensitized control element and the sensitized element, are again charged and exposed in a standard spectrophotometer to form an electrostatic charge pattern. Upon development of this charge pattern, a visible image is obtained on the sensitized zinc oxide element whereas virtually no image is visible on the nonsensitized control element.

The invention has been described in detail with particular reference to preferred embodiments thereof, but, it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A photosensitive element comprising a support having coated thereon a layer of photoconductive electrically insulating material containing therein an amount of a dithiolium salt effective to sensitize said photoconductive material.

2. In an electrophotographic element comprising a support having coated thereon a sensitized photoconductive composition, the improvement which comprises sensitizing said photoconductive composition with a sensitizer selected from the group consisting of a 1,2-dithiolium salt and a 1,3-dithiolium salt.

3. An element as in claim 2 wherein the sensitizer is selected from the group consisting of a 5-aryl-1,2-dithiolium salt, a 4- aryl-1,3-dithiolium salt, a 5-alkyll ,3-benzodithiolium salt and a 4,5-dihydro-l ,3-dithiolium salt.

4. An element as in claim 2 wherein the sensitizer is a 5-aryl- 1,2-dithiolium salt substituted at the 3-position with a radical selected from the group consisting of an alkyl radical of from one to five carbon atoms, an aryl radical and an aralkenyl radical having from two to four carbon atoms in the alkenyl moiety.

5. An element as in claim 2 wherein the sensitizer is a 4-aryl- 1,3-dithiolium salt substituted at the 2-position with a radical selected from the group consisting of an amino radical, an aryl radical, an aminoaryl radical, an aralkenyl radical and a heterocyclic alkenyl radical having at least one hetero atom selected from the group consisting of oxygen and sulfur.

6. An element as in claim 2 wherein the sensitizer is a 5- alkyl-l,3-benzodithiolium salt substituted at the 2-position with a radical selected from the group consisting of an alkyl radical of from one to five carbon atoms, an aryl radical, an aralkenyl radical having two to four carbon atoms in the alkenyl moiety and a vinyl radical.

7. An element as in claim 2 wherein the sensitizer is a 4,5- dihydro-l ,3-dithiolium salt substituted at the 2-position with a radical selected from the group consisting of an aryl radical,

an aralkenyl radical having from two to four carbon atoms in the alkenyl moiety and a heterocyclic alkenyl radical having from two to four carbon atoms in the alkenyl moiety and having at least one hetero atom selected from the group consisting of oxygen and sulfur.

8. An element as in claim 2 wherein the sensitizer is a bis-(- alkyl-1,3-benzodithiolium)alkene salt wherein the alkyl and the alkene moieties each have from 1 to 5 carbon atoms.

9. An element as in claim 2 wherein the sensitizer is 3-(4- dimethylaminophenyl)-5-phenyl-l ,Z-dithiolium perchlorate.

10. An element as in claim 2 wherein the sensitizer is 3-( 2,4- dimethoxyphenyl)-5-phenyl-l ,Z-dithiolium perchlorate.

11. An element as in claim 2 wherein the sensitizer is 2-(4- dimethylaminostyryl(-5-methyl-1,3-benzodithiolium perchlorate.

12. An element as in claim 2 wherein the sensitizer is 2-(4- diethylaminostyryl)-5-methyl-1,3-benzodithiolium perchlorate. i

13. An element as in claim 2 wherein the sensitizer is 2 [bis(4-diethylaminophenyl)vinyl]-5-methyl-l ,3-benzodithiolium perchlorate.

14. An element as in claim 2 wherein the sensitizer is 2-[2- (5-styryl-2-furyl)vinyl]-l ,3-benzodithiolium perchlorate.

15. An element as in claim 2 wherein the sensitizer is 2-(4- dimethylaminophenyl)-4-phenyl-l ,3-dithiolium perchlorate.

16. An element as in claim 2 wherein the sensitizer is 2-{p- [Nethyl-N-(2-hydroxyethyl)amino]phenyl }-4-phenyl- I ,3- dithiolium perchlorate.

17. An element as in claim 2 wherein the sensitizer is 2-(4- diphenylaminophenyl)-4-phenyl-1,3-dithiolium perchlorate.

18. An element as in claim 2 wherein the sensitizer is methane;

present in an amount of from about 0.0001 to about 30 percent by weight of the total photoconductive composition.

19. An electrophotographic element comprising a conductive support having coated thereon a layer of an organic photoconductive electrically insulating material containing an amount of a dithiolium salt effective to sensitize said photoconductive material.

20. An element as in claim 19 wherein the photoconductive composition contains a polyarylalkane photoconductor.

21. An element as in claim 19 wherein the photoconductive composition contains a photoconductor selected from the group consisting of a triphenylamine; 1,3,5-triphenylpyrazoline; 4,4'-bis(diethylamino)-2,2-dimethyltriphenyl- 2,3,4,5-tetraphenylpyrrole and 4,4- bis(diethylamino)benzophenone.

22. An element as in claim 19 wherein the sensitizer is present in an amount offrom about 0.1 to about 10 percent by weight of the total photoconductive composition.

23. An electrophotographic element comprising a support having coated thereon a layer of an electrically insulating photoconductive composition containing an organic photoconductor, an electrically insulating film-forming polymeric binder and from about 0.0001 to about 30 percent by weight of said composition of a sensitizer selected from the group consisting of a 1,2-dithiolium salt and a 1,3-dithiolium salt.

24. An element as in claim 23 wherein the sensitizer is selected from the group consisting of a 5-aryl-l,2-dithiolium salt; a 4-aryll ,3-dithiolium salt; a S-alkyl-1,3-benzodithiolium salt and a 4,5-dihydrol ,3-dithiolium salt. 

2. In an electrophotographic element comprising a support having coated thereon a sensitized photoconductive composition, the improvement which comprises sensitizing said photoconductive composition with a sensitizer selected from the group consisting of a 1,2-dithiolium salt and a 1,3-dithiolium salt.
 3. An element as in claim 2 wherein the sensitizer is selected from the group consisting of a 5-aryl-1,2-dithiolium salt, a 4-aryl-1,3-dithiolium salt, a 5-alkyl-1,3-benzodithiolium salt and a 4,5-dihydro-1,3-dithiolium salt.
 4. An element as in claim 2 wherein the sensitizer is a 5-aryl-1,2-dithiolium salt substituted at the 3-position with a radical selected from the group consisting of an alkyl radical of from one to five carbon atoms, an aryl radical and an aralkenyl radical having from two to four carbon atoms in the alkenyl moiety.
 5. An element as in claim 2 wherein the sensitizer is a 4-aryl-1,3-dithiolium salt substituted at the 2-position with a radical selected from the group consisting of an amino radical, an aryl radical, an aminoaryl radical, an aralkenyl radicAl and a heterocyclic alkenyl radical having at least one hetero atom selected from the group consisting of oxygen and sulfur.
 6. An element as in claim 2 wherein the sensitizer is a 5-alkyl-1,3-benzodithiolium salt substituted at the 2-position with a radical selected from the group consisting of an alkyl radical of from one to five carbon atoms, an aryl radical, an aralkenyl radical having two to four carbon atoms in the alkenyl moiety and a vinyl radical.
 7. An element as in claim 2 wherein the sensitizer is a 4,5-dihydro-1,3-dithiolium salt substituted at the 2-position with a radical selected from the group consisting of an aryl radical, an aralkenyl radical having from two to four carbon atoms in the alkenyl moiety and a heterocyclic alkenyl radical having from two to four carbon atoms in the alkenyl moiety and having at least one hetero atom selected from the group consisting of oxygen and sulfur.
 8. An element as in claim 2 wherein the sensitizer is a bis-(5-alkyl-1,3-benzodithiolium)alkene salt wherein the alkyl and the alkene moieties each have from 1 to 5 carbon atoms.
 9. An element as in claim 2 wherein the sensitizer is 3-(4-dimethylaminophenyl)-5-phenyl-1,2-dithiolium perchlorate.
 10. An element as in claim 2 wherein the sensitizer is 3-(2,4-dimethoxyphenyl)-5-phenyl-1,2-dithiolium perchlorate.
 11. An element as in claim 2 wherein the sensitizer is 2-(4-dimethylaminostyryl(-5-methyl-1,3-benzodithiolium perchlorate.
 12. An element as in claim 2 wherein the sensitizer is 2-(4-diethylaminostyryl)-5-methyl-1,3-benzodithiolium perchlorate.
 13. An element as in claim 2 wherein the sensitizer is 2-(bis(4-diethylaminophenyl)vinyl)-5-methyl-1,3-benzodithiolium perchlorate.
 14. An element as in claim 2 wherein the sensitizer is 2-(2-(5-styryl-2-furyl)vinyl)-1,3-benzodithiolium perchlorate.
 15. An element as in claim 2 wherein the sensitizer is 2-(4-dimethylaminophenyl)-4-phenyl-1,3-dithiolium perchlorate.
 16. An element as in claim 2 wherein the sensitizer is 2- p-(n-ethyl-n-(2-hydroxyethyl)amino)phenyl -4-phenyl-1,3-dithiolium perchlorate.
 17. An element as in claim 2 wherein the sensitizer is 2-(4-diphenylaminophenyl)-4-phenyl-1,3-dithiolium perchlorate.
 18. An element as in claim 2 wherein the sensitizer is present in an amount of from about 0.0001 to about 30 percent by weight of the total photoconductive composition.
 19. An electrophotographic element comprising a conductive support having coated thereon a layer of an organic photoconductive electrically insulating material containing an amount of a dithiolium salt effective to sensitize said photoconductive material.
 20. An element as in claim 19 wherein the photoconductive composition contains a polyarylalkane photoconductor.
 21. An element as in claim 19 wherein the photoconductive composition contains a photoconductor selected from the group consisting of a triphenylamine; 1,3,5-triphenylpyrazoline; 4,4''-bis(diethylamino)-2,2-dimethyltriphenylmethane; 2,3,4,5-tetraphenylpyrrole and 4,4''-bis(diethylamino)benzophenone.
 22. An element as in claim 19 wherein the sensitizer is present in an amount of from about 0.1 to about 10 percent by weight of the total photoconductive composition.
 23. An electrophotographic element comprising a support having coated thereon a layer of an electrically insulating photoconductive composition containing an organic photoconductor, an electrically insulating film-forming polymeric binder and from about 0.0001 to about 30 percent by weight of said composition of a sensitizer selected from the group consisting of a 1,2-dithioliuM salt and a 1,3-dithiolium salt.
 24. An element as in claim 23 wherein the sensitizer is selected from the group consisting of a 5-aryl-1,2-dithiolium salt; a 4-aryl-1,3-dithiolium salt; a 5-alkyl-1,3-benzodithiolium salt and a 4,5-dihydro-1,3-dithiolium salt. 